The disclosed invention relates to electronic devices for detecting the presence or absence of objects, and more particularly to electronic security devices for detecting the presence of intruders. In the prior art, a wide variety of security devices exist. Perhaps the most commonly used security device is a conventional lock and key. However, the lock and key is unsatisfactory in situations when one desires security where pass keys or master keys exist. This is a common situation in a hotel or motel for example. Many locks are also unsatisfactory as a security device in that they can easily be picked by burglars or vandals.
One prior art security device which overcomes these deficiencies is an acoustic detector. In operation, the acoustic detector is placed in a room to be secured. There it generates sound waves at a frequency above the audio range. These waves are reflected off objects inside the room and received by the detector to determine if any frequency shift has occurred. That is, these detectors operate on the Doppler effect. A problem with these devices however is that they only operate to detect movement inside the room which is to be secured. Accordingly, an alarm is sounded only after a burglar or vandal has actually entered the secured area.
Another prior art security device consists of an electronic alarm having a mechanically operated trigger. Typically, this device is rigidly attached to the inside of a door; and a chain connects a switch on the device to a rigid anchor point on the wall along side the door. In operation, as the door is opened, the chain tightens and throws the switch thereby activating the alarm. A problem with this device, however, is that while it detects burglars before they enter the protected area, detection still does not occur until the door is actually opened and entrance is imminent. Also, these devices are not portable; but require a permanent fixture to the door and wall inside the protected area. Thus, they are not suitable for use by travelers in hotel or motel rooms.
To overcome all of the above described problems, it is desirable to have an all electronic security device which cannot be circumvented by passkeys, cannot be picked by vandals, detects vandals before they enter the secured area, and is portable for use by travelers in motel rooms. Further, the device must be highly reliable. Specifically, the device must be immune to giving off false alarms. For example, the device's alarm must not be triggered by common everyday occurrances; such as vibrations, temperature changes, RF noise, and humidity changes. The disclosed invention meets all of the above requirements. Basically, its operation is based on electromagnetic capacitive coupling principles. Only one other known prior art attempt was made to build a security device based on capacitive coupling principles; and that work never went beyond the experimental stage, the results were never published, and no workable device resulted therefrom.
Therefore, it is one object of the invention to provide an improved security device.
Another object of the invention is to provide a security device which detects burglars, vandals, etc., before they actually enter the secured area.
Another object of the invention is to provide a security device which is portable and thereby suitable for use by travelers within motel and hotel rooms.
Another object of the invention is to provide a security device which operates without mechanical or acoustic triggering.
Another object of the invention is to provide a highly reliable security device which operates on electromagnetic capacitive coupling principles.